Low-carbon steels having a yield strength of approximately 170 megapascals (MPa) and excellent deep drawing ability are used in a variety of industries, e.g. the automobile industry. However, and despite their forming and cost advantages over high-strength steels, the relatively low-strength level results in the crash performance of such materials being mainly dependent on a thickness of a sheet thereof. As such, first generation advanced high-strength steels (AHSS) have been developed in order to reduce the weight of automotive components and thus afford for improved vehicle fuel efficiency.
In particular, dual-phase steels have been developed by subjecting low-carbon steels to an intercritical anneal followed by sufficiently rapid cooling. It is appreciated that an intercritical anneal refers to annealing the steel at a temperature or temperature range below the materials Ac3 temperature and above the Ac1 temperature where the microstructure consists of ferrite and austenite, thereby affording for the rapid cooling to transform the austenite into martensite such that a predominantly dual-phase ferrite-martensite microstructure is produced. It is also appreciated and/or known in the art that alloying elements such as manganese, chromium, molybdenum and niobium can be used to reduce the rate of cooling required for the transformation of the austenite to martensite. However, the addition of such alloying elements naturally increases the cost of the steel.
Three basic methods are known for the commercial production of dual-phase steels. First, an as-hot-rolled method produces the dual-phase microstructure during conventional hot-rolling through the control of chemistry and processing conditions. Second, a continuous annealing approach typically takes coiled hot- or cold-rolled steel strip, uncoils and anneals the steel strip in an intercritical temperature range in order to produce a ferrite plus austenite microstructure/matrix. Thereafter, sufficiently rapid cooling higher than the critical cooling rate for the steel chemistry is applied to the strip to produce the ferrite-martensite microstructure. Finally, the third method batch anneals hot- or cold-rolled material in the coiled condition.
The temperature or temperature range of the intercritical anneal is important since for a given alloy composition the intercritical anneal temperature controls or determines the amount of austenite, and its carbon content, that can be transformed to martensite. As such, high intercritical annealing temperatures have been disclosed as unsatisfactory in the prior art due to the presence of a high amount of austenite with a reduced carbon content which results in the formation of auto tempered martensite upon cooling. Previous embodiments as in U.S. Pat. No. 6,811,624 have mentioned lower temperature ranges in terms of Ac1 calculation based on the steel chemistry and combining this low temperature soak with a substantial isothermal heat treatment which is imperative to obtain predominantly ferrite-martensite structure.
However, such low soak temperatures and “substantial isothermal heat treatment” to annealed steel strip or can extend processing time of the material and thus increase costs. Therefore, an improved process for producing a coated dual-phase steel on a production scale would be desirable.